The present invention refers to an injection moulding nozzle for use in an injection moulding system, comprising a nozzle body which is provided with an essentially cylindrical outer surface and with a rear end face adapted to abut to a hot runner, further comprising at least a first and second melt passage for at least a first and second plastic melt wherein the second melt passage extends coaxially through the nozzle body and the first melt passage extends through said nozzle body at least substantially parallel to said second melt passage.
Today's injection moulding techniques make increasingly use of plastic materials which tend to show crystalline hardening structures and which, from the point of view of injection moulding technology, can only be processed with difficulties and within a narrow temperature range. It follows that a precise temperature control of the melt of plastic material along the melt passage in the injection moulding system from the melt inlet throughout the runner means and through the injection moulding nozzle up to the gate is particularly important with regard to the quality of the finished injection moulded part as well as with regard to the efficiency and the reliability of the mode of operation of the whole injection moulding system.
For a plurality of cases of use, e.g. in the field of packing industry for producing long-time packings for highly perishable foodstuff or for foodstuff which has to be used under difficult climatic conditions, it is desirable to process several plastic melts of different kinds simultaneously in one injection moulding cycle so as to obtain sandwichlike structures of materials, e.g. for providing plastic packings for foodstuff with an inner barrier layer consisting of an O.sub.2 -impervious plastic material, for embedding conductive plastic films between non-conductive plastic layers, etc. In the cae of the coinjection moulding of two plastic melts with different properties, which has to be carried out for this purpose, it is, however, difficult to master the injection moulding process from the point of view of tool technology in the case of multi-cavity hot runner systems. In particular, it is difficult to avoid, by making use of a suitable control regime, a mixing of the various plastic melts outside of the moulding cavity and to form defined core films within a basic layer of plastic material within extremely short cycle periods.
Hitherto known means for simultaneous injection moulding of different plastic melts include very often complicated injection moulding systems and associated control means, which, due to their degree of complexity, are susceptible to breakdown and expensive, and the results achieved with the aid of these means were not always satisfactory (cf. e.g. "Modern Plastics", February 1990, pages 54 to 56).
With respect to injection nozzles a plurality of different types thereof for feeding a plurality of plastic materials into a cavity are known, as for example described in EP-A 339 753, U.S. Pat. No. 4,808,101 or DE-A 36 32928. Moreover, similar injection nozzles enabling to supply of different kinds of plastic materials through separate melt channels to manufacture multilayered products are shown in EP-A 378 138, U.S. Pat. No. 4,470,936, DE-A 35 19 921 or U.S. Pat. No. 3,947,177, respectively.
A common feature of the majority of said known injection nozzles adapted to be used for coinjection moulding including sequential moulding comprises an internal structure of concentric shells or coaxial cylinders of different diameter in order to form separated flow channels for the different materials which normally should be fed to the cavity in an unmixed condition. Injection nozzles comprising a valve gated tip and a central melt bore feeding one of the melts whereas another melt flows through an annular melt towards the tip end of the nozzle are also known in the air (see for example DE-A 35 19 921). According to U.S. Pat. No. 4,470,936 two materials are fed through longitudinal separate melt channels forming melt supply bores which are disposed symmetrically with respect to a longitudinal centre axis of the nozzle.